1. Field of the Invention
It is known that photographic film has an undesirable characteristic in that a portion of the light that hits the film is used simply to activate the film and does not go into the producing of an image. An image or density is only formed on the film after this activation portion of "wasted" light has accumulated. It is also known that non-imaging light can be applied to the film immediately before, during or immediately after the imaging light so that the film will be activated and the imaging light can all be used in the formation of the image. This reduces the amount of light the camera needs to gather to produce a good image on the film.
The order in which the imaging and non-imaging light strikes the film is not critical providing both do so in very short periods of time because of the additive characteristics of photographic film. It is also known that cameras having their shutter a considerable distance in front of the film can be modified by using small lamps to shine evenly over the film to provide the non-imaging light.
Unfortunately this technique cannot be used with most of the 35 MM cameras because they have a focal plane shutter, immediately in front of the film. In order for a fixed light source to shine evenly over the film, it must be a significant distance away from the film. Since the focal plane shutter is always positioned quite close to the film, the fixed light source must be positioned such that it will shine through the shutter. Since the amount of light needed to activate a given film is fixed, the light source must be made brighter for short exposures and dimmer for long exposures. A typical range of shutter speeds for todays cameras is from 1 second to 1/1000 of a second. Therefore, a light source located ahead of the shutter would have to be variable by a factor of a thousand to be useful. None can vary by such an amount without dramatically shifting the color balance, making it impossible to use this technique for color film.
Considerable effort has gone into the development of "fast" photographic films and "fast" camera lenses in order to make it possible to take photographs in situations where the amount of light is low. It has been known for many years that photographic film requires a certain level of exposure to light before an image starts to form on the film. This activation energy of the film can be considered wasted light because it does not contribute directly to the formation of a visible image on the film. The activation energy required by a given film is constant and does not vary appreciably with the intensity of the light striking the film. Therefore, on a sunny day where there is plenty of light available, the portion of light wasted in activating the film is of no consequence because there is excess light available.
However, in situations of marginal light a large portion of the practical exposure time can be taken up in just providing the activation energy, and indeed, in certain situations it is difficult to expose the film long enough to supply enough light to the film to provide both the activation energy and the energy needed to form an image on the film. In these situations one would either not get a properly exposed image, or the exposure would be impractically long.
2. Description of the Prior Art
The patent to Herbert E. Bragg, U.S. Pat. No. 3,577,898, issued May 11, 1971, is for a supplemental illuminating of film plane in photographic cameras. This patent describes a technique for adding non-imaging light by admitting light reflected by the subject through openings or fiber optics in a manner such that the light does not pass through the camera lens. This technique has the disadvantage that the non-imaging light is variable making it difficult to admit exactly the amount of light needed to activate or sensitize the film. It also has the disadvantage that the non-image light is lowest under low light conditions when a brighter light is needed most.
The patent to Melrose R. Cole, U.S. Pat. No. 3,823,413, issued July 9, 1974, on a film speed enhancement camera, provides a non-imaging light by lamps positioned around the rear of the camera lens. While it is an improvement over the use of reflected light, the use of lamps in front of the focal plane shutter means that the intensity of the lamps must vary dramatically to admit the same amount of light to the film over a wide range of shutter speeds. This adds complexity to the camera that is undersirable and causes shifts in the color of the light source from redish in color at low light intensities to bluish at high light intensities. This will make the use of color film nearly impossible.